Zero Positive End-expiratory Pressure Before Emergence Prevents Postoperative Atelectasis.

Zero Positive End-expiratory Pressure Applied Before Emergence Preoxygenation Prevents Postoperative Atelectasis - a Randomized Controlled Trial.

A study on healthy patients undergoing anesthesia for non-abdominal surgery - evaluation of positive end-expiratory pressure versus zero positive end-expiratory pressure on the amount of atelectasis in the early postoperative period.

Positive end-expiratory pressure (PEEP) is often used during mechanical ventilation to preserve end-expiratory lung volume. After emergence and extubation, this volume will diminish instantly. Some patients will have difficulties to restore functional residual capacity (FRC) during the early phase of recovery. If routine high FIO2 has been delivered together with maintained PEEP prior to extubation, several important prerequisites are established for the development of postoperative atelectasis. The investigators hypothesize that establishing zero positive end-expiratory pressure (ZEEP) immediately prior to emergence preoxygenation, will prevent gas with high oxygen concentration from entering dorso-basal areas of the lungs, and thereby prevent postoperative atelectasis formation. This randomized controlled study will study patients undergoing non-abdominal day case surgeries under general anesthesia. The patients in the two study groups will receive mechanical ventilation with identical settings, comprising low TV, moderate PEEP, and no RM. At the end of surgery, the lungs will be examined by CT, and any atelectasis areas will be calculated. Randomization will occur after the first CT. The patients will be allocated to zero PEEP (ZEEP) or maintained PEEP during emergence preoxygenation and extubation. Importantly, the intervention group will have ZEEP established while still having low ETO2 levels, prior to any preoxygenation. The study subjects will be examined with CT no.2 approximately thirty min after extubation. Primary endpoint measure will be atelectasis area as a percentage of total lung area. Blood gases will be collected for comparison of oxygenation as a secondary outcome measure.

Atelectasis, Oxygenation, Positive end-expiratory pressure, Mechanical ventilation, General anesthesia, Computed tomography

Controlled ventilation with tidal volume of 7 mL/kg of ideal body weight and respiratory frequency 10. Fresh gas flow is set to 1 litre/min with an oxygen mixture of 40%, aiming for an inspired oxygen fraction (FiO2) of 30-35%. Positive end-expiratory pressure (PEEP) is set to 7 or 9 cm H20 (9 if BMI≥25) until start of emergence preoxygenation. Unless the patient´s SpO2 falls below 90%, the FiO2 remains unchanged throughout the procedure. First CT scan after completion of surgery, before emergence. After the first CT scan and immediately before start of emergence preoxygenation, this group will have the PEEP exchanged for zero PEEP (ZEEP). ZEEP will remain until the study subjects are extubated. Second CT scan approx. 30 min after extubation.

Controlled ventilation with tidal volume of 7 mL/kg of ideal body weight and respiratory frequency 10. Fresh gas flow is set to 1 litre/min with an oxygen mixture of 40%, aiming for an inspired oxygen fraction (FiO2) of 30-35%. Positive end-expiratory pressure (PEEP) is set to 7 or 9 cm H20 (9 if BMI≥25) even after start of emergence preoxygenation. Unless the patient´s SpO2 falls below 90%, the FiO2 remains unchanged throughout the procedure. First CT scan after completion of surgery, before emergence. After the first CT scan, this group will have PEEP remained until the study subjects are extubated. Second CT scan approx. 30 min after extubation.

Positive end-expiratory pressure remains during emergence preoxygenation and awakening and until the study subject is extubated.

Within 3 hours from induction of anesthesia. First CT after surgery, before emergence. Second CT approx 25 min after extubation. Study complete after that, no further CT.

Within 3 hours from induction of anesthesia. First blood gas after surgery, before emergence. Second approx 25 min after extubation. Study complete after that, no further blood gases.

Inclusion Criteria: American Society of Anesthesiology (ASA) class I-II Non-abdominal day case surgery under general anesthesia Exclusion Criteria: Body mass index (BMI) ≥30 kg/m2 Arterial oxygen saturation (SpO2) breathing air <95 % Chronic Obstructive Pulmonary Disease (COPD) or symptomatic asthma Symptomatic congestive heart failure Ischemic heart disease Hemoglobin <100 g/L Active smokers Active smokers and ex-smokers with a history of more than 6 pack years. Need for interscalene or supraclavicular plexus block for postoperative pain relief (risk of phrenic nerve paralysis).

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